Thanks.
What do you mean by making the test a method of TestCAPI?  I have found 
no such class.  Are you talking about the python part or the c part?

Also, atomicity in the callback is not required, as the callback is 
never interrupted implicitly.  In fact, that would make it quite 
useless.  I'll add that to the documentation.

As for your other suggestions, they're noted, I'll try having more 
threads.